1. Field of the Invention
The present invention relates to a method of and compositions for improving the friction durability of power transmission fluids.
2. Description of Related Art
Power transmission fluids, such as automatic transmission fluids, are formulated to very exacting friction requirements set by original equipment manufacturers. These requirements have two primary aspects, namely: (1) the absolute level of the friction coefficients, i.e., static friction, .mu..sub.S, and dynamic friction, .mu..sub.D, that can be achieved by these fluids, and (2) the length of time that these fluids can be used without undergoing an appreciable change in the friction coefficients. This latter performance feature is also known as friction durability.
Since friction durability is a function of the type and concentration of friction modifier molecules present in a given fluid, such as a power transmission fluid, conventionally there are only limited ways of improving friction durability. One of these ways is to add more friction modifier, i.e., to increase the concentration of friction modifier in the fluid. Since friction modifiers are consumed at a somewhat fixed rate, this will prolong the effective life of the fluid. However, this approach often is not very practical because increasing the concentration of the friction modifier usually will result in a lowering of the absolute values of the friction coefficients to a point where they are below the minimum values specified by the original equipment manufacturer. Then, as the friction modifier is consumed with time, the friction coefficients will slowly rise to unacceptable levels. The other conventional approach for improving friction durability is to find more stable friction modifiers. This is not always easy since most friction modifiers are simple organic chemicals and are subject to oxidation and chemical reactions during service.
Various compositions and methods have been suggested for modifying the properties of oleaginous fluids. For example, U.S. Pat. No. 4,253,977 relates to an ATF composition which comprises a friction modifier such as n-octadecyl succinic acid or the reaction product of an alkyl or alkenyl succinic anhydride with an aldehyde/tris hydroxymethyl aminomethane adduct and an overbased alkali or alkaline earth metal detergent. The ATF may also contain a conventional hydrocarbyl-substituted succinimide ashless dispersant such as polyisobutenyl succinimide. Other patents which disclose ATF compositions that include conventional alkenyl succinimide dispersants include, for example, U.S. Pat. Nos. 3,879,306; 3,920,562; 3,933,659; 4,010,106; 4,136,043; 4,153,567; 4,159,956; 4,596,663 and 4,857,217; British Patents 1,087,039; 1,474,048 and 2,094,339; European Patent Application 0,208,541(A2); and PCT Application WO 87/07637.
U.S. Pat. No. 3,972,243 discloses traction drive fluids which comprise gem-structured polyisobutylene oligomers. Polar derivatives of such gem-structured polyisobutylenes can be obtained by conversion of the polyisobutylene oligomers to polar compounds containing such functional groups as amine, imine, thioketone, amide, ether, oxime, maleic anhydride, etc. adducts. The polyisobutylene oligomers generally contain from about 16 to about 48 carbon atoms. Example 18 of this patent discloses reacting a polyisobutylene oil with maleic anhydride to form a polyisobutylene succinic anhydride which is useful as a detergent, as an anti-wear agent, and as an intermediate in the production of a hydrazide derivative. Other patents containing similar disclosures include, for example, U.S. Pat. No. 3,972,941; U.S. Pat. No. 3,793,203; U.S. Pat. No. 3,778,487 and U.S. Pat. No. 3,775,503.
While the prior art suggests a variety of additives for modifying the properties of various oleaginous compositions, there is no suggestion of any additives, nor of any combination of additives, which can simultaneously control the friction coefficients and friction durability of such compositions. Accordingly, there is a continuing need for new additives, as well as new methods, which would enable the formulation of oleaginous compositions, including lubricating oils and power transmission fluids, having specifically controlled friction coefficients and improved friction durability.